Analysis on the Main Controlling Factors and Characteristics of the Upper Combination of Ordovician Majiagou Formation in the Eastern Part of Sulige Gasfield

On the top of the Ordovician, the weathering crust reservoir in the upper combination of Majiagou Formation (M5₁ and M5₄) is the main gas reservoir of Lower Palaeozoic in the eastern part of Sulige gasfield. The M5₁₊₂ and M5₄¹ are the major gas production reservoir in this gasfield. Based on previous researches, the authors conducted studies including core observation, indoor thin-section analysis, scanning electron microscope and cathode luminescence. Conclusions have been drawn that the main rock type is cream powder-mud powder crystal dolomite, and the main reservoir space type is gypsum hole and karst seam; the reservoir is controlled by sedimentation, diagenesis, lava palaeo-geomorphology and tectonics; sedimentation and diagenesis are the main controlling factors, the favorable sedimentary facie is the basis of reservoir development; diagenesis determines the final formation of reservoir space; the structure of reservoir affects the reservoir communication and infiltration; the karst geomorphology controls the development degree of the reservoir space and reservoir preservation condition in a larger sense.